# Magnitude (astronomy)

magnitudemagnitudesmagabsolute magnitudeastronomical magnitude magnitudeapparent magnitudeastronomical magnitude systembrighterbrightness
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.wikipedia
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### Luminosity

luminousbolometric luminosityluminosities
Apparent magnitude depends on an object's intrinsic luminosity, its distance, and the extinction reducing its brightness.
Values for luminosity are often given in the terms of the luminosity of the Sun, L ⊙ . Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M bol ) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.

### Extinction (astronomy)

extinctionextinction factorinterstellar extinction
Apparent magnitude depends on an object's intrinsic luminosity, its distance, and the extinction reducing its brightness.
For stars that lie near the plane of the Milky Way and are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies (photometric system) is on the order of 1.8 magnitudes per kiloparsec.

### N. R. Pogson

Norman PogsonNorman R. PogsonPogson
Thus in 1856 Norman Pogson of Oxford proposed that a logarithmic scale of 5√100 ≈ 2.512 be adopted between magnitudes, so five magnitude steps corresponded precisely to a factor of 100 in brightness.
He introduced a mathematical scale of stellar magnitudes with the ratio of two successive magnitudes being the fifth root of one hundred (~2.512) and referred to as Pogson's ratio.

### Arcturus

ArcturiansArcturianArcturan
To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor. For example, Sirius is magnitude −1.46, Arcturus is −0.04, Aldebaran is 0.85, Spica is 1.04, and Procyon is 0.34.
With a near-infrared J band magnitude of −2.2, only Betelgeuse (−2.9) and R Doradus (−2.6) are brighter.

### List of brightest stars

brightest starsbrightest starone of the brightest stars
Stars that have magnitudes between 1.5 and 2.5 are called second-magnitude; there are some 20 stars brighter than 1.5, which are first-magnitude stars (see the list of brightest stars).
This is a list of the brightest stars down to magnitude +2.50, as determined by their maximum, total, or combined visual magnitudes as viewed from Earth.

### Magnitude

The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as size (magnitude means "bigness, size" ).
Magnitude (astronomy), a measure of brightness and brightness differences used in astronomy

### Absolute magnitude

Hbolometric magnitudeabsolute magnitude (H)
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude.
Absolute magnitude is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale.

### Galileo Galilei

GalileoGalileanGalilei
Astronomers from Galileo to Jaques Cassini mistook these spurious disks for the physical bodies of stars, and thus into the eighteenth century continued to think of magnitude in terms of the physical size of a star.
However, Galileo's values were much smaller than previous estimates of the apparent sizes of the brightest stars, such as those made by Tycho Brahe (see Magnitude) and enabled Galileo to counter anti-Copernican arguments such as those made by Tycho that these stars would have to be absurdly large for their annual parallaxes to be undetectable.

### Betelgeuse

α OriBetelg'''euseBetelgeuse mass loss
Betelgeuse (apparent magnitude 0.5, absolute magnitude −5.8) appears slightly dimmer in the sky than Alpha Centauri (apparent magnitude 0.0, absolute magnitude 4.4) even though it emits thousands of times more light, because Betelgeuse is much farther away.
Betelgeuse is the brightest near-infrared source in the sky with a J band magnitude of −2.99.

### Distance modulus

This is known as the distance modulus, where is the distance to the star measured in parsecs, is the apparent magnitude, and is the absolute magnitude.
It describes distances on a logarithmic scale based on the astronomical magnitude system.

### AB magnitude

AB system
One such system is the AB magnitude system, in which the reference is a source with a constant flux density per unit frequency.
The AB magnitude system is an astronomical magnitude system.

### Apparent magnitude

apparent visual magnitudemagnitudevisual magnitude
Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. For example, the human eye is more sensitive to yellow and red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude.
The brighter an object appears, the lower its magnitude value (i.e. inverse relation), with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46.

### Aldebaran

Alpha Tauribrightest starRohini
For example, Sirius is magnitude −1.46, Arcturus is −0.04, Aldebaran is 0.85, Spica is 1.04, and Procyon is 0.34.
With a near-infrared J band magnitude of −2.1, only Betelgeuse (−2.9), R Doradus (−2.6), and Arcturus (−2.2) are brighter at that wavelength.

### Intensity (physics)

intensityintensitieslight intensity
Under the modern logarithmic magnitude scale, two objects, one of which is used as a reference or baseline, whose intensities (brightnesses) measured from Earth in units of power per unit area (such as watts per square metre, W m −2 ) are
Magnitude (astronomy)

### Photographic magnitude

For example, the human eye is more sensitive to yellow and red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude.
Magnitude (astronomy)

### Astronomy

astronomicalastronomerastronomers
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Dimensionless quantity

dimensionlessdimensionless numberdimensionless quantities
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Brightness

brightintensitybrilliant
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Passband

pass bandpass-bandpassband signal
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Visible spectrum

visiblevisible lightspectrum
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Infrared

IRnear-infraredinfra-red
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths.

### Hipparchus

hipparchHipparchus of NiceaObservatory at Rhodes
The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary) who classified stars by their apparent brightness, which they saw as size (magnitude means "bigness, size" ). An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.

### Logarithmic scale

logarithmiclogarithmic unitlog
The scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512.